Materials

Description

A master is used as a mold for PDMS microchannels and valves. Fabrication of an SU-8 master requires multiples steps. First the wafer is cleaned using a piranha etch. Next, in a clean room environment, SU-8 is coated and patterned using a negative photomask. The patterned wafer is then developed, and its features verified under a microscope. Important information on SU-8 (2000) can be found here: [1].

Wafer Clean

Wafer cleaning is accomplished by means of a piranha etch. See Piranha for materials and safety procedure. For this purpose, we'll need about 40 ml to fully immerse the wafer in the cylindrical glass dish. Be sure to manipulate the wafer with caution, as it is very fragile. Use the wafer tweezers, and keep your other hand underneath in case it slips. Once pirnaha solution is prepared, let the wafer sit fully immersed for 30 min. After, carefully remove it: rinse well with distilled water, then submerge in a distilled water bath (2nd glass dish) for 5 min. Since patterning requires a dust-free environment, you may now proceed to the cleanroom. Replace the water twice more, every 5 min. Finish with a nitrogen blow dry.

Pattern Photoresist

Setup

Place both hot plates on the counter without aluminum foil. It is important that heating be uniform throughout the wafer. Make sure the aluminum-wrapped cover is also around, as it will be used to cap off the wafer as it bakes.

Use the aluminum foil to protect the top of the spin coater, and facilitate later cleaning. Turn on the power, vacuum and nitrogen feed. Set the program to A, or so as to have the following:

5s @ 500 RPM, ACL = 010
30s @ 1650 RPM, ACL = 030 (20 microns)

Turn on the ventilation switch, and open the shutter leading to the mask aligner, making sure to reach a flow level of 0.2. Turn on the mask aligner main power, and then the lamp. Set timer to 45 seconds.

Place your photomask ink-side down under the glass filter using tape. Note: If more rounded features are desired, you can place the mask ink side up. Make sure it is well centered.

10 min @ 200°C

Place slides on hot plate at 200°C for 10 minutes, then let cool 2 min. This step removes any organic solvants remaining on the surface of the wafer. The idea is to use the lid to form an oven; heating uniformity is important. Do not coat the hot plate itself with aluminum foil. For the cooling step, simply remove the wafer with the wafer tweezers, and place in the lid, covering it with aluminum foil (no contact).

Spin coat

Place the wafer at the center of the spin coater, using the alignment tool set at 100 microns. Pour the SU-8 so as to cover most of the wafer, and such as to limit the amount of bubbles.

Prebake 2 min @ 65°C + 6 min @ 95°C

Carefully place the wafer on the plates for appropriate times. Let cool 2 min at the end of the 2nd bake interval using the lid and aluminum foil.

UV Expose 45s

Place the wafer at the center of the support, and activate sample vacuum. Use the alignment XY to position the mask as the center of the wafer, and slide everything under the lamp for exposure.

Postbake 1 min @ 65°C + 3 min @ 95°C

Again, make sure to allow 2 min cooling.

Develop 4 min

Since development requires access to microscopes, it is best to move out of the cleanroom at this stage. Immerse the wafer in undiluted SU-8 developer for 4 min. Rinse with IPA, then carefully blow dry with nitrogen. Verify that development is complete by observing features under a microscope. If not, develop further.